A metal surface that contacts and undergoes relative motion with respect to another surface experiences wear. Wear is the progressive loss of material from the metal surface as a result of friction between the interacting surfaces. Excessive wear leads to premature failure of a component. Properties such as hardness are important factors that determine the wear resistance of a metal. Hardness relates to the resistance of the metal to scratching or abrasion. The higher the hardness of the metal, the greater its resistance to wear. In some cases, after fabrication of a metal component, a heat treatment operation may be performed to increase the hardness of the component surface. As a result of the heat treatment operation, a layer of material at the component surface may have a higher hardness than the bulk of the component. The increased hardness at a surface that will experience wear improves the wear resistance and prolongs the useful life of the component. Although in general, surface hardening improves wear resistance, for components that experience very high rates of wear (such as, for example, undercarriage components of a machine, ground engaging tools, TBM wear parts, etc., that are generally referred to as wear components), increased surface hardness produced by a heat treatment operation may be insufficient for a beneficial improvement in wear resistance. Such components may be hardfaced and then heat treated to further improve its wear resistance.
Hardfacing is a low cost method of depositing wear resistant surfaces on metal components to extend service life. The American Welding Society defines hardfacing as “[a] surfacing variation in which surfacing material is deposited to reduce wear.” The term surfacing is defined as “[t]he application by welding . . . of a layer, or layers, of material to a surface to obtain desired properties or dimensions, as opposed to making a joint.” AWS A3.0 Standard Welding Terms and Definitions. As opposed to a hardening heat treatment operation, which involves changing the microstructure and mechanical properties of the component surface, hardfacing involves the deposition of a new material on the base material of the component. In general, the clad material may have a similar or a different composition than the base material. Hardfacing may be performed using a number of well known welding (or cladding) techniques. These known techniques can be broadly classified into three categories as, arc welding (or arc cladding), thermal spraying, and laser-based cladding. The current disclosure is specifically directed to hardfacing of a metal component using an arc welding (or arc cladding) process.
There are a number of different arc welding techniques that are commonly used in the industry to perform hardfacing. These include, for example, gas tungsten arc welding (GTAW), plasma arc welding (PAW), plasma transferred arc (PTA), gas metal arc welding (GMAW), submerged arc welding (SAW) and several others. In these processes, an arc is established to melt the surface of the base material, usually in the presence of a shield gas. The clad material, which is introduced in either wire or powder form, is also melted by the arc to form the clad layer. Arc welding produces a clad layer that is fully welded and metallurgically bonded to the substrate of the component. This clad layer may have a higher hardness, and therefore better wear properties, than the component substrate. However, a major disadvantage of arc welding is that the high temperatures involved in depositing the clad layer act to soften (or reduce the hardness of) a layer of material on the surface of the component beneath the clad layer. This zone of heat-softened material on the component surface is referred to as the heat affected zone (HAZ). Therefore, although arc welding deposits a clad layer having high wear resistance on the component surface, the wear resistance of the underlying component surface deteriorates as a result of the heat-intensive welding process. Since the clad layer will eventually wear off after extended operation, reduced wear resistance of the underlying component surface detrimentally affects the useable life of the component by hastening component wear after the clad layer has worn off. Also, in some circumstances, a relatively soft under-layer can also cause the hard clad layer to be crushed or it can crack. The damaged clad layer will then spall off the component surface. In addition, during the cladding process, the substrate will act like a heat sink and will quench the high hardenability clad layer. This clad layer will have an as-cast untempered martensite microstructure. This untempered martensite is very hard, but it is also very brittle. When these clad components are subjected to higher impact abrasive environments, the brittle clad layer often chips and spalls.
U.S. Pat. No. 2,249,629 issued to Hopkins (the '629 patent) discloses an armored article in which an armor metal is produced by fusing together a hard metal with a base metal using electric energy discharge. After the fusing operation, the armored article is subject to heat treatment to develop the desired hardness in the hard metal and the base metal. The '629 patent disclosed steel chemistry ranges for the base material and heat treatment parameters that would yield base material hardness of 200 to 400 Brinell (approximately Rkw C 18 to 43). While the process of the '629 patent includes heat treatment after a welding operation, this process may have deficiencies. For instance, the heat affected zone created by the welding process may not be restored by the process of the '629 patent.
The disclosed hardfacing process and products are directed at overcoming these and/or other shortcomings in existing technology.